For many years, aircraft have included a backup power source which provides power for certain critical flight control or avionics systems, in the event of a failure of primary or secondary power sources, in the form of a ram air turbine driven electrical generator or pump which may be lowered from a normal stowed position within the aircraft fuselage to a deployed position in the air stream whereat the relative speed of the aircraft through the ambient air causes the ram air turbine to rotate and drive the generator or pump.
Typically, the generator or pump and ram air turbine are attached to the distal end of a strut which is typically mounted within the storage bay in the aircraft fuselage in a manner allowing the ram air turbine to be deployed by causing the strut to rotate outward into the air stream. An ejection jack or other actuation means is typically utilized to cause the outward rotation of the strut into the air stream. Locking devices are also generally provided for securing the strut in both the stowed and deployed positions. Examples of such ram air turbines and their associated deployment mechanisms are provided by U.S. Pat. Nos. 4,411,596, 4,717,095, 4,742,976, and 4,676,458, assigned to the assignee of the instant invention.
Since it is highly desirable to deploy the ram air turbine as quickly as possible when backup power is required, and also to minimize the size and weight of the ejection jack, it is common practice to mount the strut in a manner which provides rotation during deployment in a direction such that the forces generated by aerodynamic drag on the ram air turbine will aid the ejection jack in rotating the strut from the stowed to the deployed positions. Since the aerodynamic drag on the ram air turbine during deployment may be considerable for an aircraft traveling at high speed, it is often necessary when such a mounting arrangement is used to provide means for controlling the speed of deployment as well as means for absorbing energy as the strut approaches the deployed position in order to prevent damage to either the aircraft structure or the ram air turbine due to mechanical shock.
Prior ejection jacks have typically utilized a recirculating ballscrew or acme screw type actuator, driven either electromechanically or by a stored energy device such as a spring, in conjunction with a hydraulic dashpot which provides control of deployment speed and end of stroke damping to prevent mechanical shock to the ram air turbine or aircraft structure. An example of a stored energy actuator utilizing a spring and hydraulic dashpot is provided by U.S. Pat. No. 4,717,095.
While actuators of the type described above have been successfully used in the past, further improvement is required. Specifically, electromechanically driven actuators must be supplied with a source of electrical power which requires that the aircraft carry what is in essence an additional failsafe backup power supply for use in deploying another backup power supply in the form of the ram air turbine. Inclusion of this additional power supply obviously adds undesirable weight, cost, and complexity and significantly reduces the reliability of electromechanically driven actuators. Aside from the obvious problem of leakage potential created by hydraulic dashpots, an additional problem is encountered when actuators including hydraulic dashpots are utilized in the cold ambient temperatures incident with aircraft operation at high altitudes. Modern aircraft routinely operate at altitudes where ambient temperatures are -65.degree. F. to -80.degree. F. At these extreme temperatures, hydraulic damping fluids used in connection with hydraulic dashpots become so viscous that achievement of deployment within a reasonably short time becomes difficult.
An additional shortcoming of prior actuators used as ejection jacks is that they typically do not include any means for locking the strut in the stowed or deployed positions, thereby requiring the addition of separate uplock and downlock devices to be used in association with the ram air turbine.
An additional problem which must be addressed in the design of an ejection system for a ram air turbine is the provision of means for restowing the ram air turbine after the aircraft is on the ground. In this respect, the electromechanically powered actuators may have an advantage in that the motor can simply be run in the opposite direction to drive the strut back into a stowed position. For stored energy type actuators, the spring force utilized during deployment is often on the order of 2,000 pounds such that even with mechanical advantage provided by the strut, it becomes very difficult for a flightline mechanic to restow the ram air turbine manually. As a result, actuators of this type typically must incorporate means for attaching some sort of a lever or other prime mover to achieve the mechanical advantage necessary to overcome the large spring force.
The instant invention is directed to overcoming one or more of the aforementioned problems.